1. Field of the Invention
The present invention relates generally to capacitance type electroacoustical transducers and particularly to spring force biasing means for urging one member of such a device into engagement with another member of said device.
2. Description of the Prior Art
Capacitance type electroacoustical transducers are well known in the prior art. In such transducers, a diaphragm having an insulative layer and an electrically conductive layer has its insulative layer in contact with a grooved, irregular, electrically conductive surface of a substantially inflexible, plate-like member or backplate. The periphery of the diaphragm is maintained in a fixed position with respect to the transducer housing. A spring force urges said backplate into tensioning engagement with said diaphragm.
The insulative and electrically conductive layers of the diaphragm and the conductive surface of said backplate form a capacitor such that when a dc voltage is applied across the electrodes of said capacitor, irregularities on the grooved surface of the backplate set up localized concentrated electric fields in said insulative layer. When an ac signal is superimposed on said dc bias, the insulative layer is stressed such that oscillatory formations develop causing an acoustical wavefront to be propagated from the diaphragm. A received acoustical wavefront impinging on the insulative layer produces a variable voltage across the capacitor electrodes.
An extremely important design consideration for the above-described type of transducer is obtaining the correct diaphragm tension. Diaphragm tensioning greatly influences transducer acoustical output magnitude and direction, reception sensitivity and resonant frequency, for example. The prior art discloses several arrangements for obtaining the desired amount of diaphragm tensioning.
In one arrangement, illustrated in U.S. Pat. No. 3,814,864 issued June 4, 1974 to Victoreen, transducer diaphragm tensioning is provided by an adjustable coil spring. One end of the coil spring presses on the transducer backplate and the other coil spring end rests on a movable, disc shaped, spring force adjusting plate. Such an arrangement utilizes a relatively costly coil spring however, and requires an end wall to support one coil spring end.
In another arrangement, illustrated in copending U.S. Pat. application Ser. No. 741,228, by Muggli, et al., filed 12 Nov. 1976, and assigned to the assignee of the present invention, a metallic, diaphragm tensioning spring has its circular base engaging the bottom wall of a transducer housing. A plurality of fingers, having curved ends, extend upward from said circular base, said ends resiliently pressing on the diaphragm engaging backplate to provide the appropriate amount of diaphragm tensioning. This arrangement requires an end wall for spring support as in Victoreen. Also, utilizing a diaphragm tensioning spring of this type requires a relatively large amount of spring material which has an unfavorable impact on transducer cost.
Additionally, spring arrangements of the types described above require a relatively large amount of valuable space and are not readily installed in a transducer unit. Supplying an electrical signal to a transducer backplate with such spring arrangements increases transducer complexity.